This invention relates to a hydrostatic-mechanical gear unit. A gear unit of the kind with which the invention is related is described in German Application P No. 27 58 659.0-12, which is not yet published.
Hydrostatic-mechanical gear units are known in the art, for example, from the SAE publication No. 72 0724 of September 1972. In such a gear unit there is a four-shafted planet wheel gear, which functions as an integrator gear unit for the hydraulic power branch and the mechanical power branch. In known mechanisms, in the first forward driving stage and in the reverse driving stage, the drive of the output takes place purely hydrostatically. Under these conditions, the gear unit pressure angle (the product of maximum starting traction and final speed, or the product of maximum starting torque and maxiumum starting speed of the gear unit) is lower than necessary. This results in a high specific hydrostatic load, short operating life, poor shifting quality and high weight per horsepower and volume per horsepower of the unit. In addition, there is only a low reverse speed or gear unit pressure angle in the reverse stage.
German publication DE-AS No. 24 15 002 discloses another hydrostatic-mechanical gear unit which has two hydrostatic-mechanical forward stages and, depending upon the specific type of construction, two reverse stages. Here, however, it is not possible to design at low expense the gear unit with only a single reverse stage, for instance, as required by sweepers. The reverse stage is dependent upon a reversing gear with two shifting couplings built into the mechanical power branch. The gear unit must thus, as a matter of principle, thus be designed with four couplings, which brings about equal forward and reverse driving conditions. The couplings for the first and second stages are designed a head of the integrator gear unit in the hydraulic power branch. Consequently, the total speed of the different shaft components of the integrator gear unit is higher, with the result that whereby the component parts are subject to a higher load and are noiser. In general, a necessary consequence of such a gear unit is that one shaft component of the integrator gear unit rotates in the final gearing together with the multiple ones at the speed of the driving motor or the speed of the shaft which is connected with the drive shaft. In DE-AS No. 24 15 002, this shaft has, in the starting range, to transfer the starting torque so it must accordingly be constructed as a cross-piece or internal gear wheel taking up a relatively large volume.
Further disadvantages include the unfavorable efficiency due to higher shaft losses, in particular through the reversing gear unit in the mechanical power branch which is shifted in between there, the more unfavorable design, and the weight and volume per horsepower.
The gear unit described above in Patent Application No. P 27 58 659.0-12 provides a number of advantages over the mechanisms known in the art. Included in these advantages are that a continuous speed control is possible over at least two forward drive stages, whereby in the first stage the gear unit pressure angle corresponds to double the hydrostatic pressure angle.
In each operating condition there is a hydraulic-mechanical power transfer with a small proportion being hydraulic power (hydraulic power on the average being below 30 percent) and thus there is particularly good efficiency. Additionally, high gear unit pressure angle or power yield of the hydrostatic gear unit can be obtained, thereby providing smaller weight and volume per horsepower. Alternatively, small, economical hydrostatic units can be used.
There is also a low load on the hydrostatic gear unit; low hydraulic idle power; and high shifting quality, since there is an extensive range of continuous traction and efficiency in shifting. The starting power can be greater than the hydrostatic pressure angle. The well-known disadvantage of the initial breakaway torque of the hydromotor is eliminated, since both hydrostatic units at the starting point are in rotation. This brings about a further heightening of the starting tractions and, in addition, better starting characteristics.
Only one, not both, hydrostatic unit to be controlled, the construction can be compactly adapted to the vehicle, and the greatest torque occurs in the planet carrier which means that there is a small specific tooth load and leading to a more economical construction.
The present invention is directed to providing a power-branched gear unit which has a more compact construction, is better suited to the vehicle, and with a lower gear unit weight and volume, provides a high conversion range or gear unit pressure angle. Additionally, the gear unit construction can with these features be adapted for different types of vehicles--such as sweepers, construction machines, commercial vehicles and possibly buses.